In recent times, coordinate measuring apparatus are utilized in increasing numbers directly in the manufacturing area and are interlinked with the processing machines such as a flexible manufacturing system.
In such interlinked systems, it is not possible to carry out length measurements on the workpiece at a reference temperature of 20.degree. C. The workpiece has undergone a change in temperature because of the just completed manufacturing process which includes washing and drying operations. This other temperature must be determined and considered in arriving at the measuring result.
The determination of the workpiece temperature has been carried out manually up to now in that operating personnel placed a temperature sensor on the workpiece and the temperature value would then be read off and would be inputted into the computer of the coordinate measuring apparatus.
German Patent 3,013,378 discloses a method wherein the processed workpieces are delivered automatically to a measuring device equipped with a temperature sensor. The workpiece is measured with respect to its dimensions and its temperature. For obtaining control data for the machining process, the measuring values are compared with corresponding values of a reference workpiece which is always measured anew at regular intervals.
The known method is, however, poorly suited if workpieces having different geometries are to be automatically measured. For this situation, a complex manipulating system is required to bring the temperature sensor into contact with the workpiece.
It is necessary to detect the workpiece temperature to 0.1.degree. K. for precise length measurements. Known temperature sensors which operate without contact have up to now not been able to detect temperatures with this precision. For temperature detection with high precision, precision resistors or thermistors are mostly utilized which, however, must be connected to an electronic circuit for the generation of measurement data and also must be connected to a voltage supply. The required cable connections required for this purpose constitute a disturbance in the material flow systems.
Furthermore, the precision resistors utilized for the temperature measurement have a relatively long time constant. After such a temperature sensor is mounted on a workpiece, it then takes several minutes until the exact temperature value of the workpiece can be read off. This time constant causes standstill periods of the coordinate measuring apparatus.